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Abstract

There is considerable evidence for ongoing, late-stage interaction between the magmatic system at
Merapi volcano, Indonesia, and local crustal carbonate (limestone). Calc-silicate xenoliths within
Merapi basaltic-andesite eruptives display textures indicative of intense interaction between magma
and crustal carbonate, and Merapi feldspar phenocrysts frequently contain individual crustally
contaminated cores and zones. In order to resolve the interaction processes between magma and
limestone in detail we have performed a series of time-variable de-carbonation experiments in
silicate melt, at magmatic pressure and temperature, using a Merapi basaltic-andesite and local
Javanese limestone as starting materials. We have used in-situ analytical methods to determine the
elemental and strontium isotope composition of the experimental products and to trace the textural,
chemical, and isotopic evolution of carbonate assimilation. The major processes of magmacarbonate
interaction identified are: i) rapid decomposition and degassing of carbonate, ii)
generation of a Ca-enriched, highly radiogenic strontium contaminant melt, distinct from the
starting material composition, iii) intense CO2 vesiculation, particularly within the contaminated
zones, iv) physical mingling between the contaminated and unaffected melt domains, and v)
chemical mixing between melts. The experiments reproduce many of the features of magmacarbonate
interaction observed in the natural Merapi xenoliths and feldspar phenocrysts. The Carich,
high 87Sr/86Sr contaminant melt produced in the experiments is considered as a pre-cursor to
the Ca-rich (often “hyper-calcic”) phases found in the xenoliths and the contaminated zones in
Merapi feldspars. The xenoliths also exhibit micro-vesicular textures which can be linked to the
CO2 liberation process seen in the experiments. This study, therefore, provides well-constrained
petrological insights into the problem of crustal interaction at Merapi and points toward the
substantial impact of such interaction on the volatile budget of the volcano.